The effects of Batrachochytrium dendrobatidis and sub-lethal nitrate concentrations on Xenopus laevis

Abstract

The pathogenic fungus Batrachochytrium dendrobatidus (Bd) is responsible for mass disruption of amphibian species and causes the disease known as chytridiomycosis which disrupts the amphibian skin resulting in high mortality rates (Pessier et al. 1999). There are limited studies of the internal and immune functions of frogs during Bd exposure exist and studies aimed at understanding the effects of Bd exposure combined with anthropogenic stressors such as nitrate are limited as well. The aim of this study was to test treatment combinations involving nitrate concentration (0, 40 and 80 mg/L) and pre- and post-metamorphic exposure to Batrachochytrium dendrobatidis (Bd). Melanomacrophages (MMPs) were used as biomarkers to understand how treatment combinations impact Xenopus laevis internally. Statistical analyses were used to determine host survivability, mean MMP counts and to correlate frog performance variables with MMP count. To assure Bd treatment was effective PCR was used to detect presence of Bd in specimens. Results indicate negative impacts in groups exposed to 40mg/L nitrate and individuals exposed to Bd after metamorphosis including decreased survivability, indicating that interactions cause high mortality of individuals. Individuals exposed to Bd as tadpoles in 40 mg/L nitrate survived longer, suggesting that early exposure to Bd can result in longer survival time. Frogs with early exposure to Bd resulted in longer survival time. Higher stress was associated with 40 mg/L nitrate and no exposure to Bd after metamorphosis, implicating 40 mg/L nitrate as the cause for higher MMP numbers. High numbers of MMPs in frogs exposed to Bd in the 0 mg/L nitrate groups suggests Bd does stress the frog internally. Surprisingly, groups in the 80 mg/L concentration had fewer MMPs and typically longer lifespans and proportion of individuals surviving, suggesting mitigating effects for frogs exposed to Bd, likely through interaction between Bd and nitrate.